1. Field of the Invention
The present invention relates to a machine tool.
2. Description of the Related Art
A machine tool such as a numerical control lathe (NC lathe) is conventionally used to manufacture desired precision parts by performing front-machining and back-machining operations on a workpiece.
FIG. 12 is a schematic illustration showing a conventional NC lathe 1. The NC lathe 1 includes: a headstock 3 having a main spindle 2 chucking a workpiece W for a front-machining operation; and a backworking attachment 4 opposed to the headstock 3 in a direction of the main spindle 2 (in a direction of Z-axis). The backworking attachment 4 includes a sub spindle 5 for chucking the workpiece W for a back-machining operation. The NC lathe 1 includes a tool spindle stock 7 having a tool spindle 6. The main spindle 2 and the sub spindle 5 is under rotation control about Z-axis (C1-axis control) by a computer (not shown) according to a predetermined program. The tool spindle stock 7 is under movement control in the direction of Z-axis and also in directions perpendicular to the direction of Z-axis (in a direction of X-axis or Y-axis). The tool spindle 6 is under rotation control about the axis of the tool spindle 6 (C2-axis control). The tool spindle stock 7 is also under pivoting control about the direction of Y-axis (B-axis control)
In the configuration described above, a front-machining operation is performed on the workpiece W chucked by the main spindle 2 with a tool 8 attached to the tool spindle 6. The workpiece W is detached from the main spindle 2 and then chucked by the sub spindle 5. A back-machining operation is performed on the workpiece W chucked by the sub spindle 5 with the tool 8 attached to the tool spindle 6. The tool 8 used for a back-machining operation may be different from the tool used for a front-machining operation. The direction of Y-axis shown in FIG. 12 is perpendicular to a plane of paper.
In a known vertical machining center, a tool and a workpiece chucking unit are detachably attached to a main spindle by using an automatic tool changer (ATC), with the workpiece chucking unit being co-rotatable with the main spindle. (see JP-B-2760749).
Further, in another known machine tool, a workpiece clumping unit is attached to a pivotable tool spindle stock by using an ATC, and a workpiece is detachably attached to a headstock by the clumping unit (see JP-A-2002-263909).
In another known secondary machining device in an automatic lathe, an L-shaped pivoting unit has, at each protruding portion: a tool spindle for automatically mounting a secondary machining tool for performing a front-machining operation on a workpiece chucked by a main spindle; and a sub spindle for performing a back-machining operation on the workpiece that was cut off after the front-machining operation was performed (see JP-A-1-240201).
The following problems may be encountered in the related art described above.
In the NC lathe 1 shown in FIG. 12, the backworking attachment 4 is provided to perform a back-machining operation. Therefore, a size of the NC lathe 1 in the direction of Z-axis must be a total of the lengths in the longitudinal direction of the headstock 3, the tool spindle stock 7 and the backworking attachment 4, to which a total of the lengths of the workpiece W and the tool 8 is further added. Therefore, the device is very large.
In JP-B-2760749 described above, movement of the main spindle is limited to rotation around the axis and travelling in the directions of X, Y and Z. Therefore, in order to set a workpiece on a rotary table, the axis rotation of which is directed in the horizontal direction, it is necessary to prepare another chucking unit different from the chucking unit of the main spindle, capable of rotating the workpiece in the vertical and horizontal directions. Therefore, the apparatus cost is raised. Since movement of the main spindle is limited as described above, it is difficult to operate precise machining on an inclined face of the workpiece. Therefore, a machining ability is not sufficient.
In JP-A-2002-263909 described above, an elbow-shaped workpiece is shown, whose center of axis at both ends are different from each other. The elbow-shaped workpiece can be delivered by a clamping member attached to the tool spindle stock from a first headstock to a second headstock in such manner that the center of axis of the workpiece may be coaxially aligned with the headstock. However, a machining operation is always performed under the condition that the workpiece is attached to the first headstock and the second headstock. Therefore, in the same manner as that of the conventional NC lathe 1 shown in FIG. 12, the machine becomes very large. In addition to that, it is necessary to provide two headstocks and a tool spindle stock. Accordingly, the apparatus cost is raised.
In JP-A-1-240201 described above, in order to perform a front-machining operation (a secondary machining operation in a front-machining operation) and a back-machining operation on a workpiece, it is necessary to provide an L-shaped pivoting unit having a sub spindle and a tool spindle stock respectively formed in the protruding ends. Therefore, the apparatus becomes large in its configuration and further the cost is raised.
The present invention has been accomplished in view of the above problems. An object of an embodiment of the present invention is to provide a machine tool capable of performing various machining operations including a back-machining operation on a workpiece in a smaller apparatus configuration at a lower apparatus cost.